Biyani's Think Tank
Concept based notes
Applied Electronics
(B.Tech)
Apurva Vashishth
Asst. Professor
Deptt. of Engineering
Biyani International Institute of Engineering and Technology
2
Published by :
Think Tanks
Biyani Group of Colleges
Concept & Copyright :
Biyani Shikshan Samiti
Sector-3, Vidhyadhar Nagar,
Jaipur-302 023 (Rajasthan)
Ph : 0141-2338371, 2338591-95 Fax : 0141-2338007
E-mail : acad@biyanicolleges.org
Website :www.gurukpo.com; www.biyanicolleges.org
Edition : 2013
Price :
While every effort is taken to avoid errors or omissions in this Publication, any mistake or
omission that may have crept in is not intentional. It may be taken note of that neither the
publisher nor the author will be responsible for any damage or loss of any kind arising to
anyone in any manner on account of such errors and omissions.
Leaser Type Setted by :
Biyani College Printing Department
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Preface
I
am glad to present this book, especially designed to serve the needs of the students.
The book has been written keeping in mind the general weakness in understanding the
fundamental concepts of the topics. The book is self-explanatory and adopts the “Teach
Yourself” style. It is based on question-answer pattern. The language of book is quite easy and
understandable based on scientific approach.
Any further improvement in the contents of the book by making corrections, omission and
inclusion is keen to be achieved based on suggestions from the readers for which the author
shall be obliged.
I acknowledge special thanks to Mr. Rajeev Biyani, Chairman & Dr. Sanjay Biyani, Director
(Acad.) Biyani Group of Colleges, who are the backbones and main concept provider and also
have been constant source of motivation throughout this Endeavour. They played an active role
in coordinating the various stages of this Endeavour and spearheaded the publishing work.
I look forward to receiving valuable suggestions from professors of various educational
institutions, other faculty members and students for improvement of the quality of the book. The
reader may feel free to send in their comments and suggestions to the under mentioned
address.
Note:
A feedback form is enclosed along with think tank. Kindly fill the feedback form
and submit it at the time of submitting to books of library, else NOC from
Library will not be given.
Author
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Syllabus
Unit 1: Feedback Amplifiers
Classification, Feedback concept, Transfer gain with feedback, General characteristics of
negative feedback amplifiers. Analysis of voltage-series, voltage-shunt, current-series
and current-shunt feedback amplifier. Stability criterion.
Unit 2: Oscillators
Classification. Criterion for oscillation. Tuned collector, Hartley, Colpitts, RC Phase shift,
Wien bridge and crystal oscillators, Astable, monostable and bistable multivibrators.
Schmitt trigger. Blocking oscillators.
Unit 3: High Frequency Amplifiers
Hybrid Pi model, conductances and capacitances of hybrid Pi model, high frequency
analysis of CE amplifier, gain-bandwidth product. Emitter follower at high frequencies.
Unit 4: Tuned Amplifier
Band Pass Amplifier, Parallel resonant Circuits, Band Width of Parallel resonant circuit.
Analysis of Single Tuned Amplifier, Primary & Secondary Tuned Amplifier with BJT &
FET. Double Tuned Transformer Coupled Amplifier. Stagger Tuned Amplifier. Pulse
Response of such Amplifier. Shunt Peaked Circuits for Increased Bandwidth.
Unit 5: Power Amplifiers
Power amplifier circuits, Class A output stage, class B output stage and class AB output
stages, class C amplifiers, pushpull amplifiers with and without transformers.
Complementary symmetry & quasi complimentary symmetry amplifiers
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5
Unit 1
Feedback Amplifier
Feedback
Q.1
Ans
Explain feedback amplifier?
The process of injecting a fraction of output energy of the same device back to the
input is known as feedback.
Thus whenever the output has an influence on the effective input to an amplifier,
the amplifier becomes a feedback amplifier.
It is also known as the closed-loop amplifier because the signal path through
A- and F-networks forms a loop.
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There are two types of feedback in amplifiers:
1. Positive feedback - They are also known as regenerative feedback. The feedback
signal is in phase with the input signal.
2. Negative feedback- they are also known as degenerative feedback. The feedback
signal is out of phase with the output signal.
Q.2
Ans
Explain positive and negative feedback.
Positive feedback
When the feedback energy is in phase with the input signal and this aids its, it is
called positive feedback.
The positive feedback increases the gain of the amplifier.
The distortion and instability increases.
Both the amplifier and feedback network introduce a phase shift of 180 , which
results in 360 around the loop, causing the feedback in phase with input signal.
This means that the feedback signal will add to or "regenerate" the input signal.
The result is a larger amplitude output signal than would occur without the
feedback.
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Negative feedback
When the feedback signal is out of phase with the input signal. The feedback
signal is 180
out of phase with the input signal.
It reduces the gain of the amplifier.
Increases the bandwidth and higher cut-off frequency.
Decreases the lower cut-off frequency and also the noise.
Reduces the distortion.
Provide stability in gain.
Improves the frequency response of amplifier.
The amplifier introduces a phase shift of 180 into the circuit while feedback
circuit does not provide any phase shift i.e. 0 phase shift, thus the feedback
signal will be 180 out of phase with input signal.
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Q.3
Ans
Negative feedback circuit
Give different classification of amplifiers
The amplifier without negative feedback is known as basic amplifier. There gain
is unstable and output impedence do not meet the requirenment of general
applications.
Amplifiers may be classified into 4 types1. Voltage amplifier- they are intended to amplify an input voltage signal and
provide an output voltage signal. The voltage amplifier is generally a
voltage controlled voltage source.
Input impedance is required to be high and output impedance is required to be
low.
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Voltage gain, A v = V o / Vin
2. Current amplifier- the input signal of a current amplifier is essentially a
current.
The current amplifier is essentially a current-controlled current source.
The ideal current amplifier is defined as an amplifier which provides an output
current proportional to the signal current and proportionality constant is
independent of magnitudes of source and load resistances.
Current gain, Ai = I o / I in
3. Transconductance amplifier- here the input signal is voltage and its output
signal is a current. The ideal transconductance amplifier provides an output
current which is proportional to the signal voltage and the proportionality
constant is independent of the magnitudes of resistances.
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G= I L / V 1
G is the proportionality constant of transconductance amplifier.
4. Trnsresistance amplifier- here, the input signal is current and the output
signal is voltage. The ideal trnsresistance amplifier provides an output
voltage in proportion to the signal current, and the proportionality constant
is independent of the magnitudes of resistance (source and load). In ideal
conditions, it should have zero input and output resistances.
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Applied Electronics
Q.4
Ans
11
What are the different terms used in feedback concept?
Different terms used in feedback concept are:
1. Signal source- it is either a signal voltage or signal current in series or in
parallel with resistance respectively.
2. Feedback network- it is in the form of passive two port network and be
formed of resistor, inductor and capacitors. Its function is to return a function
of the output energy to the input of the amplifier,
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3. Sampling network- it is of two types, voltage and current sampling network
output voltage is sampled by current connecting the feedback network either
shunt across the output or in series with the output.
4. Comparator or mixer- a differential amplifier , which has two inputs and ane
output proportional to difference between the signals at the two inputs, is
usually used as mixer or comparator. It is again of two types, series and shunt
mixers.
Q.5
Ans
Explain different types of feedback connections?
Different types of feedback connections are
•
•
•
If the feedback network samples the output voltage, it is voltage feedback. If it
samples the output current, it is current feedback.
The feedback signal can be connected in series or in parallel with the signal
source and the amplifier input terminals, so called series feedback and parallel
feedback.
So, there are four types of negative feedback in amplifier circuits:
 Series voltage feedback
 Series current feedback
 Parallel voltage feedback
 Parallel current feedback
 In voltage feedback, the input terminals of the feedback network
are in parallel with the load, and the output voltage appears at
the input terminals of the feedback block.
 Whereas in current feedback, the input terminals of the feedback
network are in series with the load, and the load current flows
through the input of the feedback block.
 As a result, a simple test on the feedback type is to open-circuit
or short-circuits the load. If the feedback signal vanishes for an
open-circuit load, then it is current feedback. If the feedback
signal vanishes for a short-circuit load, it is voltage feedback.
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Q.6
Ans
15
Give analyses of voltage series , voltage shunt, current serirs , current shunt.
1. Voltage amplifier (series-shunt)
Series input connection increase input resistance – avoid loading effects on the
input signal source.
Shunt output connection decrease the output resistance - avoid loading effects on
the output signal when output load is connected.
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Avf
Vo
Vi
R
1 2
R1
,
Vo
AvV
V
Vi V fb
R1
V fb
Avf
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1
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Vi
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Av
1
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Ri (1
Av
AvV
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2. Current amplifier (shunt-series)-
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Basic current amplifier with input resistance, Ri and an open-loop current gain,
Ai.
Current IE is the difference between input signal current and feedback current.
Feedback circuit samples the output current – provide feedback signal in shunt
with signal current.
Increase in output current – increase feedback current – decrease error current.
Smaller error current – small output current – stabilize output signal.
Shunt input connection decrease input resistance – avoid loading effects on the
input signal current source.
Series output connection increase the output resistance - avoid loading effects on
the output signal due to load connected to the amplifier output.
Q.7
Give the stability criterion for amplifier.
Ans
VOUT/VIN = Aol / (1+ Aolβ)
If: Aolβ = -1
Then: VOUT/VIN = Aol / 0  ∞
If VOUT/VIN = ∞  Unbounded Gain
Any small changes in VIN will result in large changes in VOUT which will
feed back to VIN and result in even larger changes in VOUT  OSCILLATIONS 
INSTABILITY !!
Aolβ: Loop Gain
Aolβ = -1  Phase shift of +180°, Magnitude of 1 (0dB)
fcl: frequency where Aolβ = 1 (0dB)
Stability Criteria:
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At fcl, where Aolβ = 1 (0dB), Phase Shift < +180°
Desired Phase Margin (distance from +180° Phase Shift) > 45°
The Nyquist Stability Criterion is a unique method for determining stability of a
closed loop system. A closed loop system is stable if all of the closed loop poles
are in the left half of the s-plane. That's a very basic fact about a system. What's
peculiar is that Nyquist showed that it is possible to get information about closed
loop pole location by plotting open loop frequency response data. Now, if you
think about it, it might seem peculiar that open loop frequency response behavior
would let you glean that kind of information about closed loop time response
behavior. Yet, peculiar as it might seem, that's the way it is, and Nyquist's result the Nyquist Stability Criterion - is widely used in the design and analysis of
control systems everywhere.
The Nyquist diagram is basically a plot of G(j* w) where G(s) is the open-loop
transfer function and w is a vector of frequencies which encloses the entire righthalf plane. In drawing the Nyquist diagram, both positive and negative
frequencies (from zero to infinity) are taken into account. We will represent
positive frequencies in red and negative frequencies in green. The frequency
vector used in plotting the Nyquist diagram usually looks like this (if you can
imagine the plot stretching out to infinity):
However, if we have open-loop poles or zeros on the jw axis, G(s) will not be defined at
those points, and we must loop around them when we are plotting the contour. Such a
contour would look as follows:
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Please note that the contour loops
around the pole on the jw axis. As
we mentioned before, the Matlab
nyquist command does not take
poles or zeros on the jw axis into
account and therefore produces
an incorrect plot.
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Unit 2
Oscillators
Q.1
What is an oscillator? Give classification of oscillators.
Ans
Oscillator - An electronic oscillator may be define in any one of the following four
ways:
i)
ii)
iii)
iv)
It is a circuit which convert dc energy into an energy at a very high
frequency
It is an electronic source of alternating current or voltage having sine,
square or saw tooth or pulse shapes
It is circuit which generates an ac output signals without requiring any
externally applied input signal.
It is an unstable amplifier.
Classification of Oscillators
The electronic oscillators may be broadly classified into the following two
categories.
The oscillators, which provide an output having a sine wave form, are called
sinusoidal or harmonic oscillators. Such oscillators can provide output at
frequencies ranging from 20 Hz to GHz.
1. Sinusoidal or Harmonic Oscillators
1. Tuned Circuit Oscillators
These oscillators use a tuned-circuit consisting of inductors (L) and
capacitors (C) and are used to generate high frequency signals. Thus they
are also known as radio frequency (CRT) oscillators. Such oscillators are
Hartley and Colpitts oscillators etc.
2. RC Oscillators
These oscillators use resistors and capacitors and are used to generate low
or audio-frequency signals. Thus they are also known as audio-frequency
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(A.F) oscillators. Such oscillators are phase-shift and wien-bridge
oscillators.
3. Crystal Oscillators
These oscillators use quartz crystals and are used to generate highly
stabilized output signal with frequencies up to 10 Mhz. The pierce
oscillator is an example of a crystal oscillator.
4. Negative-resistance Oscillators
These oscillators use negative-resistance characteristic of the devices such
as tunnel diodes. A tuned diode oscillator is an example of negative
resistance oscillator.
2. Non-sinusoidal or Relaxation Oscillators
The oscillators, which provide an output having a square, rectangular or saw
tooth waveform, are called non-sinusoidal or relaxation oscillators. Such
oscillators can provide output at frequencies ranging from zero to 20 Mhz.
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Q.2
What is Barkhausen criterion for feedback oscillator?
Ans
The overall gain of a feedback amplifier is Af = A/(1 + Aβ), where A is the gain of
the internal amplifier, β is the feedback ratio, and −Aβ is the loop gain. For
positive feedback, the feedback gain is expressed as:
For Aβ = 1, the eq.yields Af = ∞. The amplifier then produces an output voltage
without any externally applied input voltage. Thus, the amplifier becomes an
oscillator. When the signal equals Vo′ ⇒ AβVo = Vo or Aβ = 1, the output voltage
regenerates itself and the amplifier oscillates. This condition is called the
Barkhausen criterion. This condition means that | Aβ | = 1 and the phase angle of
Aβ is zero or an integral multiple of 360. The basic conditions for oscillation in a
feedback amplifier are: (1) the feedback must be regenerative, (2) the loop-gain
must be unity, and (3) the phase difference must be zero or an integral multiple of
360.
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Applied Electronics
Vs
+
23
V
Vo
A(f)
+
Vf
SelectiveNetwork
(f)
Thus, the condition for sinusoidal oscillation of frequency f0 is;
A jω0 β jω0
1
This is known as Barkhausen criterion.
Q.3
Explain LC network?
Ans
Some sine-wave oscillators use resonant circuits consisting of inductance and
capacitance. For example, recall the tank circuit in which a resonant circuit stores
energy alternately in the inductor and capacitor, producing a sine wave. If there
were absolutely no internal resistances in a tank circuit, oscillations would
continue indefinitely, Each resonant circuit does, however, contain some
resistance which dissipates power. This power loss causes the amplitude to
decrease; The reduction of amplitude in an oscillator circuit is referred to as
DAMPING. Damping is caused by both tank and load resistances. The larger the
tank resistance, the greater the amount of damping. Loading the tank causes the
same effect as increasing the internal resistance of the tank. The effect of this
damping can be overcome by applying regenerative feedback.
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Resonant Frequency of a LC Oscillator
Where:
L is the Inductance in Henries
C is the Capacitance in Farads
ƒr is the Output Frequency in Hertz
This equation shows that if either L or C is decreased, the frequency increases.
This output frequency is commonly given the abbreviation of ( ƒr ) to identify it
as the "resonant frequency".
To keep the oscillations going in an LC tank circuit, we have to replace all the
energy lost in each oscillation and also maintain the amplitude of these
oscillations at a constant level. The amount of energy replaced must therefore be
equal to the energy lost during each cycle. If the energy replaced is too large the
amplitude would increase until clipping of the supply rails occurs. Alternatively,
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if the amount of energy replaced is too small the amplitude would eventually
decrease to zero over time and the oscillations would stop.
Two well-known Oscillators:
• Colpitts Oscillator
• Harley Oscillator
Q.4
Ans
Give circuit diagram with explanation of Hartley oscillator?
In the Hartley Oscillator the tuned LC circuit is connected between the collector
and the base of the transistor amplifier. As far as the oscillatory voltage is
concerned, the emitter is connected to a tapping point on the tuned circuit coil.
The feedback of the tuned tank circuit is taken from the centre tap of the inductor
coil or even two separate coils in series which are in parallel with a variable
capacitor, C as shown.
The Hartley circuit is often referred to as a split-inductance oscillator because coil
L is centre-tapped. In effect, inductance L acts like two separate coils in very close
proximity with the current flowing through coil section XY induces a signal into
coil section YZ below. An Hartley Oscillator circuit can be made from any
configuration that uses either a single tapped coil (similar to an autotransformer)
or a pair of series connected coils in parallel with a single capacitor as shown
below.
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Basic Hartley Oscillator Circuit
When the circuit is oscillating, the voltage at point X (collector), relative to point Y
(emitter), is 180o out-of-phase with the voltage at point Z (base) relative to point
Y. At the frequency of oscillation, the impedance of the Collector load is resistive
and an increase in Base voltage causes a decrease in the Collector voltage. Then
there is a 180o phase change in the voltage between the Base and Collector and
this along with the original 180o phase shift in the feedback loop provides the
correct phase relationship of positive feedback for oscillations to be maintained.
The amount of feedback depends upon the position of the "tapping point" of the
inductor. If this is moved nearer to the collector the amount of feedback is
increased, but the output taken between the Collector and earth is reduced and
vice versa. Resistors, R1 and R2 provide the usual stabilizing DC bias for the
transistor in the normal manner while the capacitors act as DC-blocking
capacitors.
In this Hartley Oscillator circuit, the DC Collector current flows through part of
the coil and for this reason the circuit is said to be "Series-fed" with the frequency
of oscillation of the Hartley Oscillator being given as.
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Note: LT is the total cumulatively coupled inductance if two separate coils are
used including their mutual inductance, M.
The frequency of oscillations can be adjusted by varying the "tuning" capacitor, C
or by varying the position of the iron-dust core inside the coil (inductive tuning)
giving an output over a wide range of frequencies making it very easy to tune.
Also the Hartley Oscillator produces output amplitude which is constant over
the entire frequency range.
Q.5
Ans
What do you mean by RC phase shift oscillator?
In an RC Oscillator circuit the input is shifted 180o through the amplifier stage
and 180o again through a second inverting stage giving us "180o + 180o = 360o" of
phase shift which is the same as 0o thereby giving us the required positive
feedback. In other words, the phase shift of the feedback loop should be "0".
In a Resistance-Capacitance Oscillator or simply an RC Oscillator, we make use
of the fact that a phase shift occurs between the input to a RC network and the
output from the same network by using RC elements in the feedback branch
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The circuit on the left shows a single resistor-capacitor network and whose output
voltage "leads" the input voltage by some angle less than 90 o. An ideal single-pole
RC circuit would produce a phase shift of exactly 90o, and because 180o of phase
shift is required for oscillation, at least two single-poles must be used in an RC
oscillator design. However in reality it is difficult to obtain exactly 90 o of phase
shift so more stages are used. The amount of actual phase shift in the circuit
depends upon the values of the resistor and the capacitor, and the chosen
frequency of oscillations with the phase angle ( Φ ) being given as:
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The basic RC Oscillator which is also known as a Phase-shift Oscillator,
produces a sine wave output signal using regenerative feedback obtained from
the resistor-capacitor combination. This regenerative feedback from the RC
network is due to the ability of the capacitor to store an electric charge, (similar to
the LC tank circuit).
This resistor-capacitor feedback network can be connected as shown above to
produce a leading phase shift (phase advance network) or interchanged to
produce a lagging phase shift (phase retard network) the outcome is still the same
as the sine wave oscillations only occur at the frequency at which the overall
phase-shift is 360o. By varying one or more of the resistors or capacitors in the
phase-shift network, the frequency can be varied and generally this is done by
keeping the resistors the same and using a 3-ganged variable capacitor.
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Q.6
Explain colpitts oscillator?
Ans
The basic configuration of the Colpitts Oscillator resembles that of the Hartley
Oscillator but the difference this time is that the centre tapping of the tank subcircuit is now made at the junction of a "capacitive voltage divider" network
instead of a tapped autotransformer type inductor as in the Hartley oscillator.
Colpitts Oscillator Circuit
The Colpitts oscillator uses a capacitor voltage divider as its feedback source. The
two capacitors, C1 and C2 are placed across a common inductor, L as shown so
that C1, C2 and L forms the tuned tank circuit the same as for the Hartley
oscillator circuit.
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The advantage of this type of tank circuit configuration is that with less self and
mutual inductance in the tank circuit, frequency stability is improved along with
a more simple design.
As with the Hartley oscillator, the Colpitts oscillator uses a single stage bipolar
transistor amplifier as the gain element which produces a sinusoidal output.
Consider the circuit below.
Basic Colpitts Oscillator Circuit
The transistor amplifiers emitter is connected to the junction of capacitors, C1 and
C2 which are connected in series and act as a simple voltage divider. When the
power supply is firstly applied, capacitors C1 and C2 charge up and then
discharge through the coil L. The oscillations across the capacitors are applied to
the base-emitter junction and appear in the amplified at the collector output.
The amount of feedback depends on the values of C1 and C2 with the smaller the
values of C the greater will be the feedback.
The required external phase shift is obtained in a similar manner to that in the
Hartley oscillator circuit with the required positive feedback obtained for
sustained un-damped oscillations. The amount of feedback is determined by the
ratio of C1 and C2 which are generally "ganged" together to provide a constant
amount of feedback so as one is adjusted the other automatically follows. The
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frequency of oscillations for a Colpitts oscillator is determined by the resonant
frequency of the LC tank circuit and is given as:
where CT is the capacitance of C1 and C2 connected in series and is given as:.
The configuration of the transistor amplifier is of a Common Emitter Amplifier
with the output signal 180o out of phase with regards to the input signal. The
additional 180o phase shift require for oscillation is achieved by the fact that the
two capacitors are connected together in series but in parallel with the inductive
coil resulting in overall phase shift of the circuit being zero or 360o.
Resistors, R1 and R2 provide the usual stabilizing DC bias for the transistor in the
normal manner while the capacitor acts as DC-blocking capacitors. The radiofrequency choke (RFC) is used to provide a high reactance (ideally open circuit) at
the frequency of oscillation, ( ƒr ) and a low resistance at DC.
Q.7
Discuss the wein bridge oscillator?
Ans
The Wien Bridge Oscillator is so called because the circuit is based on a
frequency-selective form of the Whetstone bridge circuit. The Wien Bridge
oscillator is a two-stage RC coupled amplifier circuit that has good stability at its
resonant frequency, low distortion and is very easy to tune making it a popular
circuit as an audio frequency oscillator but the phase shift of the output signal is
considerably different from the previous phase shift RC Oscillator.
The Wien Bridge Oscillator uses a feedback circuit consisting of a series RC
circuit connected with a parallel RC of the same component values producing a
phase delay or phase advance circuit depending upon the frequency. At the
resonant frequency ƒr the phase shift is 0o
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The output of the operational amplifier is fed back to both the inputs of the
amplifier. One part of the feedback signal is connected to the inverting input
terminal (negative feedback) via the resistor divider network of R1 and R2 which
allows the amplifiers voltage gain to be adjusted within narrow limits. The other
part is fed back to the non-inverting input terminal (positive feedback) via the RC
Wien Bridge network.
The RC network is connected in the positive feedback path of the amplifier and
has zero phase shift a just one frequency. Then at the selected resonant frequency,
( ƒr ) the voltages applied to the inverting and non-inverting inputs will be equal
and "in-phase" so the positive feedback will cancel out the negative feedback
signal causing the circuit to oscillate.
Also the voltage gain of the amplifier circuit MUST be equal to three "Gain = 3"
for oscillations to start. This value is set by the feedback resistor network, R1 and
R2 for an inverting amplifier and is given as the ratio -R1/R2. Also, due to the
open-loop gain limitations of operational amplifiers, frequencies above 1MHz are
unachievable without the use of special high frequency op-amps.
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Q.8
Briefly describe the crystal oscillator .
Ans
To obtain a very high level of oscillator stability a Quartz Crystal is generally
used as the frequency determining device to produce another types of oscillator
circuit known generally as a Quartz Crystal Oscillator.
When a voltage source is applied to a small thin piece of quartz crystal, it begins
to change shape producing a characteristic known as the Piezo-electric effect.
This piezo-electric effect is the property of a crystal by which an electrical charge
produces a mechanical force by changing the shape of the crystal and vice versa, a
mechanical force applied to the crystal produces an electrical charge.
Then, piezo-electric devices can be classed as Transducers as they convert energy
of one kind into energy of another (electrical to mechanical or mechanical to
electrical). This piezo-electric effect produces mechanical vibrations or oscillations
which are used to replace the LC tank circuit in the previous oscillators. There are
many different types of crystal substances which can be used as oscillators with
the most important of these for electronic circuits being the quartz minerals
because of their greater mechanical strength.
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The quartz crystal used in a Quartz Crystal Oscillator is a very small, thin piece
or wafer of cut quartz with the two parallel surfaces metallised to make the
required electrical connections. The physical size and thickness of a piece of
quartz crystal is tightly controlled since it affects the final frequency of
oscillations and is called the crystals "characteristic frequency". Then once cut and
shaped, the crystal cannot be used at any other frequency. In other words, its size
and shape determines its frequency.
The crystals characteristic or resonant frequency is inversely proportional to its
physical thickness between the two metallised surfaces. A mechanically vibrating
crystal can be represented by an equivalent electrical circuit consisting of low
resistance, large inductance and small capacitance as shown below.
The equivalent circuit for the quartz crystal shows an RLC series circuit, which
represents the mechanical vibrations of the crystal, in parallel with a capacitance,
Cp which represents the electrical connections to the crystal. Quartz crystal
oscillators operate at "parallel resonance", and the equivalent impedance of the
crystal has a series resonance where Cs resonates with inductance, L and a
parallel resonance where L resonates with the series combination of Cs and Cp as
shown.
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Crystal Reactance
The slope of the reactance against frequency above, shows that the series
reactance at frequency ƒs is inversely proportional to Cs because below ƒs and
above ƒp the crystal appears capacitive, i.e. dX/dƒ, where X is the reactance.
Between frequencies ƒs and ƒp, the crystal appears inductive as the two parallel
capacitances cancel out. The point where the reactance values of the capacitances
and inductance cancel each other out Xc = XL is the fundamental frequency of the
crystal.
A quartz crystal has a resonant frequency similar to that of a electrically tuned
tank circuit but with a much higher Q factor due to its low resistance, with typical
frequencies ranging from 4kHz to 10MHz. The cut of the crystal also determines
how it will behave as some crystals will vibrate at more than one frequency. Also,
if the crystal is not of a parallel or uniform thickness it have two or more resonant
frequencies having both a fundamental frequency and harmonics such as second
or third harmonics.
Generally though the fundamental frequency is much more stronger or
pronounced than the harmonics around it so this is the one used. The equivalent
circuit above has three reactive components and there are two resonant
frequencies, the lowest is a series type frequency and the highest a parallel type
resonant frequency.
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We have seen in the previous tutorials, that an amplifier circuit will oscillate if it
has a loop gain greater or equal to one and the feedback is positive. In a Quartz
Crystal Oscillator circuit the oscillator will oscillate at the crystals fundamental
parallel resonant frequency as the crystal always wants to oscillate when a
voltage source is applied to it.
Q.9
Explain monostable, bistable and astable multivibrator ?
Ans
Astable Multivibrator
Astable Multivibrator is a two stage switching circuit in which the output of the
first stage is fed to the input of the second stage and vice versa. The outputs of
both the stages are complementary. This free running multivibrator generates
square wave without any external triggering pulse. The circuit has two states and
switches back and forth from one state to another, remaining in each state for a
time depending upon the discharging of a capacitor through a resistor.
The following circuit is an astable multivibrator, or oscillator. The two transistors
are cross-coupled in such a way that the circuit switches back and forth between
two states. In one state, the base of Q1 is about one diode drop above ground,
allowing a base current to flow. This keeps Q1 switched on, in saturation mode,
allowing a current to flow through the collector, keeping Q1's collector voltage
low, and discharging C1. Q2 is switched off, because its base voltage is not high
enough to switch it on
As the collector current into Q1 charges C1, the base voltage for Q2 goes up, until
it is high enough to switch on Q2, causing a current to flow through its collector,
which drops the collector voltage (the current causes a voltage drop across the
resistor above it). The right side of C2 has dropped, but the voltage across it hasn't
changed, so this causes Q1's base voltage to drop below ground, switching it off.
Then we get the other half of the cycle, with current flowing through Q2. This
continues until Q1 turns on, and then the cycle repeats.
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Monostable multivibrator
In a monostable multivibrator, one of the state is absolutely permanent i.e, stable
and the other one is temporary i.e, quasi-stable. When an external trigger pulse is
applied to the mono-stable at appropriate point, the mono-stable changes it state
from stabe state to quasi-stable state. It stays in the quasi-stable state for a
predetermined length of certain interval remains there until another pulse is
applied. Thus a mono stable multivibrator can not generate square wave of its
own like an astable multivibrator. Only external pulse will cause if to generate the
square wave.
In other words, a multi vibrator in which one transistor is always conducting
(i.e. in the ON state) and the other is non conducting (i.e. in the OFF state) is
called mono stable multivibrator. It is also called a single shot or single swing or a
one shot multi vibrator. Other names are delay multi-vibrator and univibrator.
To change the monostble multivibrator state from the stable to quasi-state the
external trigger pulses are to be applied. In general the negative triggering has
greater sensitivity, because here the negative pulse amplitude should be enough,
so as to bring the operating point from saturation to active region. Secondly when
the base emitter voltage of a junction changes from forward bias to reverse bias,
its input impedance is continuously rising, which avoids the loading of the
triggering source. It should be further noted that the monostable period is
affected by this method.
The positive pulse triggering has sensitivity, because to turn of the transistor from
the OFF state, it is necessary to feed the excess stored charge in the base such that
the amplitude of triggering pulse is enough and is derived from a low impedance
source, which can supply a peak demand current to turn on.
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Bistable multivibrator
The bistable multivibrator has two absolutely stable states. It will remain in
whichever state it happens to be until a trigger pulse causes it to switch to the
other state. For instance, suppose at any particular instant, transistor Q1 is
conducting and transistor Q2 is at cut-off. If left to itself, the bistable multivibrator
will stay in this position for ever. However, if an external pulse is applied to the
circuit in such a way that Q1 is cut-off and Q2 is turned on, the circuit will stay in
the new position. Another trigger pulse is then required to switch the circuit back
to its original state.
In other words a multivibrator which has both the state stable is called a bistable
multivibrator. It is also called flip-flop, trigger circuit or binary. The output pulse
is obtained when, and why a driving (triggering) pulse is applied to the input. A
full cycle of output is produced for every two triggering pulses of correct polarity
and amplitude.
Figure (a) shows the circuit of a bistable multivibrator using two NPN transistors.
Here the output of a transistor Q2 is coupled put of a transistor Q1 through a
resistor R2. Similarly, the output of a transistor Q1 is coupled to the base of
transistor Q2 through a resistor R1. The capacitors C2 and C1 are known as speed
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up capacitors. Their function is to increase the speed of the circuit in making
abrupt transition from one stable state to another stable state. The base resistors
(R3 and R4) of both the transistors are connected to a common source (-VBB). The
output of a bistable multivibrator is available at the collector terminal of the both
the transistor Q1 and Q2. However, the two outputs are the complements of each
other.
Let us suppose, if Q1 is conducting, then the fact that point A is at nearly ON
makes the base of Q2 negative (by the potential divider R2 - R4) and holds Q2 off.
Similarly with Q2 OFF, the potential divider from VCC to -VBB (RL2, R1, R3) is
designed to keep base of Q1 at about 0.7V ensuring that Q1 conducts. It is seen
that Q1 holds Q2 OFF and Q2 hold Q1 ON.
Suppose, now a positive pulse is applied momentarily to R. It will cause Q2 to
conduct. As collector of Q2 falls to zero, it cuts Q1 OFF and consequently, the
BMV switches over to its other state.
Similarly, a positive trigger pulse applied to S will switch the BMV back to its
original state.
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Q.10 What do you mean by Schmitt trigger ?
Ans
Schmitt trigger belongs to a class of bistable multivibrator circuits. In a bistable,
there exist two D.C. couplings from each output to input of the other. But in
Schmitt trigger circuit, there exists only one coupling. It can be recalled that if in
the emitter coupled bistable the feedback network from the collector of transistor
Q2 to the base of transistor Q1 is removed , it becomes a Schmitt trigger circuit.
The Schmitt trigger is used for wave shaping circuits. It can be used for
generation of a square wave from a sine wave input. Basically, the circuit has two
opposite operating states like in all other multivibrator circuits. However, the
trigger signal is not, typically, a pulse waveform but a slowly varying A.C.
Voltage. The Schmitt trigger is level sensitive and switches the output state at two
distinct trigger levels. One of the triggering levels is called a lower trigger level
and the other as upper trigger level .
The circuit of a Schmitt trigger, the circuit of Schmitt trigger contains of two
identical transistors Q1 and Q2 coupled through an emitter RE. The resistor R1 and
R2 form a voltage divider across the VCC supply and ground. These resistors
provide a small forward bias on the base of transistor Q2.
Let us suppose that initially there is no signal at the input. Then as soon as the
power supply VCC is switched on, the transistor Q2 starts conducting. The flow of
its current through resistor RE produces a voltage drop across it. This voltage
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drop acts as a reverse bias across the emitter junction of transistor Q 1 due to
which it cuts-off. As a result of this, the voltage at its collector rises to V CC. This
rising voltage is coupled to the base of transistor Q2 through the resistor R1. It
increases the forward bias at the base of transistor Q2 and therefore drives it into
saturation and holds it there. At this instant, the collector voltage, level are V C1 =
VCC and VC2 = VCE(sat).
Now suppose an A.C. signal is applied at the input of the Schmitt trigger (i.e. at
the base of the transistor Q1). As the input voltage increases above zero, nothing
will happen till it crosses the upper trigger level (U.L.T). As the input voltage
increases, above the upper trigger level, the transistor Q 1 conducts. The point, at
which it starts conducting, is known as upper trigger point (U.T.P). As the
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transistor Q1 conducts, its collector voltage falls below VCC. This fall is coupled
through resistor R1 to the base of transistor Q2 which reduces its forward bias.
This in turn reduces the current of transistor Q2 and hence the voltage drop across
the resistor RE. As a result of this, the reverse bias of transistor Q 1 is reduced and
it conducts more. As the transistor Q1 conducts more heavily, its collector further
reduces due to which the transistor Q1 conducts near cut-off. This process
continues till the transistor Q1 is driven into saturation and Q2 into cut-off. At this
instant, the collector voltage levels are VC1 = VCE(sat) and VC2 = VCC as shown in the
figure.
The transistor Q1 will continue to conduct till the input voltage falls below the
lower trigger level (L.T.L). It will be interesting to know that when the input
voltage becomes equal to the lower trigger level, the emitter base junction of
transistor Q1 becomes reverse biased. As a result of this, its collector voltage starts
rising toward VCC. This rising voltage increases the forward bias across transistor
Q2 due to which it conducts. The point, at which transistor Q2 starts conducting, is
called lower trigger point (L.T.P). Soon the transistor Q 2 is driven into saturation
and Q1 to cur-off. This completes one cycle. The collector voltage levels at this
instant are VC1 = VCC and VC2 = VCE(sat). No change in state will occur during the
negative half cycle of the input voltage.
It is proved from the above discussion that the output of a Schmitt trigger is a
positive going pulse, whose width depends upon the time during which
transistor Q1 is conducting. The conduction time is set by the upper and lower
trigger levels.
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Unit 3
High Frequency Amplifier
Q.1
Give the need of high frequency amplifier ?
Ans
In the high- and intermediate-frequency assemblies of telecommunication
systems, amplifiers composed of discrete transistors are still used in addition to
modern integrated amplifiers. This is particularly the case in high-frequency
power amplifiers employed in transmitters. In low-frequency assemblies, on the
other hand, only integrated amplifiers are used. The use of discrete transistors is
due to the status quo of semiconductor technology.
The development of new semiconductor processes with higher transit frequencies
is soon followed by the production of discrete transistors, but the production of
integrated circuits on the basis of a new process does not usually occur until some
years later. Furthermore, the production of discrete transistors with particularly
high transit frequencies often makes use of materials or processes which are not
(or not yet) suitable for the production of integrated circuits in the scope of
production engineering or for economic reasons.
The high growth rate in radio communication systems has, however, boosted the
development of semiconductor processes for high-frequency applications.
Integrated circuits on the basis of compound semiconductors such as galliumarsenide (GaAs) or silicongermanium (SiGe) can be used up to the GHz range.
For applications up to approximately 3 GHz bipolar transistors are mainly used,
which, in the case of GaAs or SiGe designs, are known as hetero-junction bipolar
transistors (HBT). Above 3 GHz, gallium-arsenide junction FETs or metalsemiconductor field effect transistors (MESFETs) are used.1 the transit frequencies
range between 50 . . . 100 GHz.
Q.2
Give the principal construction of high frequency amplifier?
Ans
In principle, integrated high-frequency amplifiers use the same circuitry as lowfrequency or operational amplifiers. A typical amplifier consists of a differential
amplifier used as a voltage amplifier and common-collector circuits used as
current amplifiers or impedance converters (see Fig. 27.1a). The differential
amplifier is often designed as a cascode differential amplifier to reduce its reverse
transmission and its input capacitance (no Miller
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45
effect).. Since the transit frequency ofhigh-frequency transistors (fT ≈ 50 . . . 100
GHz) is approximately 100 times higher than that of low-frequency transistors (fT
≈ 500 MHz. . . 1 GHz), the bandwidth of the amplifier increases by approximately
the same factor. This, however, presumes that the parasitic of the bond wires and
the connections within the integrated circuit can be reduced enough so that the
bandwidth is primarily determined by the transit frequency of the transistors and
is not limited by the connections. This is a key problem in both the design and use
of high-frequency semiconductor processes.
Q.3
Explain hybrid pie model ?
Ans
The hybrid-pi model is a popular circuit model used for analyzing the small
signal behavior of bipolar junction and field effect transistors. The model can be
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46
quite accurate for low-frequency circuits and can easily be adapted for higher
frequency circuits with the addition of appropriate inter-electrode capacitances
and other parasitic elements.
Combine the internal capacitances and lead resistance in a modified Hybrid-π
model.
B
ic (ω )
ib (ω )
r
C
+
v
-
gmv
C
C
+
vce
-
ro
Therefore use this model to construct small-signal circuit when vi is operating at
high frequency.
E
* Note , since ,Zc = 1/jωc all currents and voltages will be dependent on operating
frequency
and the voltage across r is v , but v
vbe .
Note at low-frequencies, the model reverts to the original Hybrid-π model.
Q.4
Ans
Explain differential high frequency amplifier ?
This device is a monolithic two-stage high- frequency amplifier with differential
inputs and outputs.
Internal feedback provides wide bandwidth, low phase distortion, and excellent
gain stability. Variable gain based on signal summation provides large AGC
control over a wide bandwidth with low harmonic distortion. Emitter-follower
outputs enable the device to drive capacitive loads. All stages are current-source
biased to obtain high common-mode and supply-voltage rejection ratios. The gain
may be electronically attenuated by applying a control voltage to the AGC pin.
No external compensation components are required.
This device is particularly useful in TV and radio IF and RF AGC circuits, as well
as magnetic-tape and disk-file systems where AGC is needed. Other applications
include video and pulse amplifiers where a large AGC range, wide bandwidth,
low phase shift, and excellent gain stability are required.
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Q.5
Ans
47
Explain impedance matching with series inductance?
For high-frequency bipolar transistors with a transit frequency above 10 GHz, the
capacitances of the actual transistor are so low that the input and output
capacitances are formed by the parasitic capacitance of the case. The equivalent
circuit for these transistors with case capacitances CBE and CCE and case
inductances LB, LC and LE where the relationships are CBE > CCE > CC and LB ≈
LC > LE. The equivalent circuit can be simplified owing to the component
dimensions. When using the simplified equivalent circuit for a multi-stage
amplifier, the circuitry between each of the stages represents a Collins filter. The
capacitances of the filter are formed by the capacitances of the transistor and the
inductances of the filter by the series connection of the case inductances and an
external inductance. Therefore, if the dimensions are favorable, matching between
the stages can be achieved with a series inductance. Similarly, the parasitic
elements of the transistors at the input and output of the amplifier can be
integrated into a Collins filter.
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Unit 4
Tuned Amplifier
Q.1
What do you understand by tuned amplifier?
Ans
'Tuned' amplifiers are amplifiers involving a resonant circuit, and are intended
for selective amplification within a narrow band of frequencies. Radio and TV
amplifiers employ tuned amplifiers to select one broadcast channel from among
the many concurrently induced in an antenna or transmitted through a cable.
Q.2
Ans
Give the advantages of tuned amplifier?
(i) Small power loss.
A tuned parallel circuit employs reactive components Land C. Consequently, the
power loss in such a circuit is quite low. On the other hand, if a resistive load is
used in the collector circuit, there will be considerable loss of power. Therefore,
tuned amplifiers are highly efficient.
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(ii) High selectivity.
A tuned circuit has the property of selectivity i.e. it can select the desired
frequency for amplification out of a large number of frequencies simultaneously
impressed upon it. For instance, if a mixture of frequencies including fr is fed to
the input of a tuned amplifier, then maximum amplification occurs for fr. For all
other frequencies, the tuned circuit offers very low impedance and hence these
are amplified to a little extent and may be thought as rejected by the circuit. On
the other hand, if we use resistive load in the collector, all the frequencies will be
amplified equally well i.e. the circuit will not have the ability to select the desired
frequency.
(iii) Smaller collector supply voltage.
Because of little resistance in the parallel tuned circuit, it requires small collector
supply voltage VCC. On the other hand, if a high load resistance is used in
the collector for amplifying even one frequency, it would mean large voltage drop
across it due to zero signal collector current. Consequently, a higher collector
supply will be needed.
Q.3 Explain the quality factor Q and also the bandwidth in tuned amplifier?
Ans
Quality factor Q
The Q, quality factor, of a resonant circuit is a measure of the “goodness” or
quality of a resonant circuit. A higher value for this figure of merit corresponds to
a narrower bandwidth, which is desirable in many applications. More formally, Q
is the ration of power stored to power dissipated in the circuit reactance and
resistance.
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The sharp rtesonance curve means that impedance should fall very rapidly as the
frequency is varied from resonance frequency.
Thus the ratio of inductive reactance of the coil at resonance is known as quality
factor. This is expressed by Q and is given by
Q = Xl / R
Q = ωr L/ R
Q = 2π fr L / R
Band width
The rang of frequencies at which the impedance of the tuned amplifier falls to
70.7% of the maximum impedance is called its bandwidth.
Bandwidth = f2 – f1 = fr /Q
The difference f1 – f2 is called as pass band of the circuit . the frequencies below f1
and above f2 will not pass. So the bandwidth gives the discriminating property of
the resonant circuit. Therefore, higher is the Q value, smaller is the bandwidth.
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Q.4
Ans
Give analyses of single tuned amplifier.
A single tuned amplifier consists of a transistor amplifier containing a parallel
tuned circuit as the collector load. The values of capacitance and inductance of the
tuned circuit are so selected that its resonant frequency is equal to the frequency
to be amplified
Operation.
The high frequency signal to be amplified is given to the input of the amplifier.
The resonant frequency of parallel tuned circuit is made equal to the frequency of
the signal by changing the value of C. Under such conditions, the tuned circuit
will offer very high impedance to the signal frequency. Hence a large output
appears across the tuned circuit. In case the input signal is complex containing
many frequencies, only that frequency which corresponds to the resonant
frequency of the tuned circuit will be amplified. All other frequencies will be
rejected by the tuned circuit. In this way, a tuned amplifier selects and amplifies
the desired frequency.
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Q.5
Ans
53
What do you mean by double tuned amplifier?
The disadvantage of potential instability in single tuned amplifiers can be
overcome in Double tuned amplifiers. A double tuned amplifier consists of
inductively coupled two tuned circuits. One L1, C1 and the other L2, C2. In the
Collector terminals change in the coupling of the two tuned circuits results in
change in the shape of the Frequency response curve. By proper adjustment of the
coupling between the two coils of the two tuned circuits, the required results
(High selectivity, high Voltage gain and required bandwidth) may be obtained.
Operation
The high Frequency signal to be amplified is applied to the input terminal of the
amplifier. The resonant Frequency of TUNED CIRCUIT connected in the
Collector circuit is made equal to signal Frequency by varying the value of C1.
Now the tuned circuit L1, C1 offers very high Impedance to input signal
Frequency and therefore, large output is developed across it. The output from the
tuned circuit L1,C1 is transferred to the second tuned circuit L2, C2 through
Mutual Induction. Hence the Frequency response in Double Tuned amplifier
depends on the Magnetic Coupling of L1 and L2.
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Advantage
Tuned amplifiers are usually used in Radio Frequency stage of wireless
communication systems, where such circuits are assigned the work of selecting
the desired carrier frequency and of amplifying the permitted pass band around
the selected carrier frequency.
ie; The Tuned amplifiers should be highly Selective. But high selectivity requires a
Tuned circuit with a high Quality Factor.
A high Quality Factor (Q Factor) circuit will give a high Voltage gain, but at the
same time, it will give much reduced bandwidth because the bandwidth is
Inversely Proportional to Q-Factor. It means a tuned amplifier with reduced
bandwidth may not be able to amplify equally the complete band of signals and
results in Poor Reproduction of input signal. This is called potential instability in
tuned amplifiers.
Q.6
Ans
Explain staggered tuned amplifier .
Stagger Tuned Amplifiers are used to improve the overall frequency response of
tuned Amplifiers. Stagger tuned Amplifiers are usually designed so that the
overall response exhibits maximal flatness around the centre frequency.
It needs a number of tuned circuit operating in union. The overall frequency
response of a Stagger tuned amplifier is obtained by adding the individual
response together. Since the resonant Frequencies of different tuned circuits are
displaced or staggered, they are referred as staggered tuned amplifier.
The main advantage of stagger tuned amplifier is increased bandwidth. Its
Drawback is Reduced Selectivity and critical tuning of many tank circuits. They
are used in RF amplifier stage in Radio Receivers.
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Unit 5
Power Amplifier
Q.1
What do you mean by power amplifier ?
Ans
The Small Signal Amplifier is generally referred to as a "Voltage" amplifier
because they usually convert a small input voltage into a much larger output
voltage. Sometimes an amplifier circuit is required to drive a motor or feed a
loudspeaker and for these types of applications where high switching currents are
needed Power Amplifiers are required.
As their name suggests, the main job of a "Power Amplifier" (also known as a
large signal amplifier), is to deliver power to the load, and as we know from
above, is the product of the voltage and current applied to the load with the
output signal power being greater than the input signal power. In other words, a
power amplifier amplifies the power of the input signal which is why these types
of amplifier circuits are used in audio amplifier output stages to drive
loudspeakers.
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The power amplifier works on the basic principle of converting the DC power
drawn from the power supply into an AC voltage signal delivered to the load.
Although the amplification is high the efficiency of the conversion from the DC
power supply input to the AC voltage signal output is usually poor.
The perfect or ideal amplifier would give us an efficiency rating of 100% or at
least the power "IN" would be equal to the power "OUT". However, in reality this
can never happen as some of the power is lost in the form of heat and also, the
amplifier itself consumes power during the amplification process.
Q.2
Give the classification of power amplifier?
Ans
Type of Signal
Type of
Configuration
Classification
Frequency of
Operation
Small Signal
Common
Emitter
Class A Amplifier
Direct Current (DC)
Large Signal
Common Base
Class B Amplifier
Audio Frequencies (AF)
Common
Collector
Class AB Amplifier
Radio Frequencies (RF)
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Class C Amplifier
Q.3
VHF, UHF and SHF
Frequencies
Explain the different terms used in power amplifier ?
Ans
1. Collector efficiency- the ratio of a.c. output to the zero signal power is defiend as
collector efficiency.
This is expressed as (η).
η = average a.c. power output / average dc power input
2. Power dissipation capacity- It is termed as the ability of a power transistor to
dissipate the heat developed in it.
Durng operation of transistor . it heats up due to large current . the increased
tmpreture influences the operating point. So the transistor must dessipate the
heat to the surroundinds.
3. Distortion – the change of output wave shape from the input wave shape of the
amplifier is defined as distortion.it occurs as transistor is a non linear device.
Q.4
Explain the different classes of amplifier.
Ans
The classification of an amplifier as either a voltage or a power amplifier is made
by comparing the characteristics of the input and output signals by measuring the
amount of time in relation to the input signal that the current flows in the output
circuit. We saw in the Common Emitter transistor tutorial that for the transistor to
operate within its "Active Region" some form of "Base Biasing" was required. This
small Base Bias voltage added to the input signal allowed the transistor to
reproduce the full input waveform at its output with no loss of signal.
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However, by altering the position of this Base bias voltage, it is possible to
operate an amplifier in an amplification mode other than that for full waveform
reproduction. With the introduction to the amplifier of a Base bias voltage,
different operating ranges and modes of operation can be obtained which are
categorized according to their classification. These various mode of operation are
better known asAmplifier Class.
Audio power amplifiers are classified in an alphabetical order according to their
circuit configurations and mode of operation. Amplifiers are designated by
different classes of operation such as class "A", class "B", class "C", class "AB", etc.
These different Amplifier Classes range from a near linear output but with low
efficiency to a non-linear output but with a high efficiency. No one class of
operation is "better" or "worse" than any other class with the type of operation
being determined by the use of the amplifying circuit. There are typical maximum
efficiencies for the various types or class of amplifier, with the most commonly
used being:
Class A Amplifier - has low efficiency of less than 40% but good signal
reproduction and linearity.
Class B Amplifier - is twice as efficient as class A amplifiers with a
maximum theoretical efficiency of about 70% because the amplifying
device only conducts (and uses power) for half of the input signal.
Class AB Amplifier - has an efficiency rating between that of Class A and
Class B but poorer signal reproduction than class A amplifiers.
Class C Amplifier - is the most efficient amplifier class as only a very
small portion of the input signal is amplified therefore the output signal
bears very little resemblance to the input signal. Class C amplifiers have
the worst signal reproduction.
Q.5
What is class A amplifier ?
Ans
Class A Amplifier operation is where the entire input signal waveform is
faithfully reproduced at the amplifiers output as the transistor is perfectly biased
within its active region, thereby never reaching either of its Cut-off or Saturation
regions. This then results in the AC input signal being perfectly "centred" between
the amplifiers upper and lower signal limits as shown below.
Class A Output Waveform
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In this configuration, the Class A amplifier uses the same transistor for both
halves of the output waveform and due to its biasing arrangement the output
transistor always has current flowing through it, even if there is no input signal.
In other words the output transistors never turn "OFF". This results in the class A
type of operation being very inefficient as its conversion of the DC supply power
to the AC signal power delivered to the load is usually very low.
Generally, the output transistor of a Class A amplifier gets very hot even when
there is no input signal present so some form of heat sinking is required. The DC
current flowing through the output transistor (Ic) when there is no output signal
will be equal to the current flowing through the load. Then a pure Class A
amplifier is very inefficient as most of the DC power is converted to heat.
Q.6
Explain class B amplifier?
Ans
Class B
Unlike the Class A amplifier mode of operation above that uses a single transistor
for its output power stage, the Class B Amplifier uses two complimentary
transistors (an NPN and a PNP) for each half of the output waveform. One
transistor conducts for one-half of the signal waveform while the other conducts
for the other or opposite half of the signal waveform. This means that each
transistor spends half of its time in the active region and half its time in the cut-off
region thereby amplifying only 50% of the input signal.
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Class B operation has no direct DC bias voltage like the class A amplifier, but
instead the transistor only conducts when the input signal is greater than
the base-emitter voltage and for silicon devices is about 0.7v. Therefore, at zero
input there is zero output. This then results in only half the input signal being
presented at the amplifiers output giving a greater amount of amplifier efficiency
as shown below.
Class B Output Waveform
In a class B amplifier, no DC current is used to bias the transistors, so for the
output transistors to start to conduct each half of the waveform, both positive and
negative, they need the base-emitter voltageVbe to be greater than the 0.7v
required for a bipolar transistor to start conducting. Then the lower part of the
output waveform which is below this 0.7v window will not be reproduced
accurately resulting in a distorted area of the output waveform as one transistor
turns "OFF" waiting for the other to turn back "ON". The result is that there is a
small part of the output waveform at the zero voltage cross over point which will
be distorted. This type of distortion is called Crossover Distortion and is looked at
later on in this section.
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Q.7
What do you mean by class AB and class C amplifier?
Ans
Class AB
The Class AB Amplifier is a compromise between the Class A and the Class B
configurations above. While Class AB operation still uses two complementary
transistors in its output stage a very small biasing voltage is applied to the Base of
the transistor to bias it close to the Cut-off region when no input signal is
present.An input signal will cause the transistor to operate as normal in its Active
region thereby eliminating any crossover distortion which is present in class B
configurations. A small Collector current will flow when there is no input signal
but it is much less than that for the Class A amplifier configuration. This means
then that the transistor will be "ON" for more than half a cycle of the waveform.
This type of amplifier configuration improves both the efficiency and linearity of
the amplifier circuit compared to a pure Class A configuration.
Class AB Output Waveform
The class of operation for an amplifier is very important and is based on the
amount of transistor bias required for operation as well as the amplitude required
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for the input signal. Amplifier classification takes into account the portion of the
input signal in which the transistor conducts as well as determining both the
efficiency and the amount of power that the switching transistor both consumes
and dissipates in the form of wasted heat.
Class C
A class C power amplifier is biased to operate for less than 180° of the input
signal cycle, as shown in circuit. The tuned circuit in the output, however, will
provide a full cycle of output signal for the fundamental or resonant frequency of
the tuned circuit (L and C tank circuit) of the output. The use of such amplifiers
is, therefore, limited for a fixed frequency, as occurs in communication circuits,
for example. operation of a class C circuit is not intended primarily for large
signal or power amplifiers.
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Q,8
Exlain push pull amplifier with transistor ?
Ans
Class A push pull :
A push pull amplifier can be made in Class A, Class B, Class AB or Class C
configurations. The circuit diagram of a typical Class A push pull amplifier is
shown above. Q1 and Q2 are two identical transistor and their emitter terminals
are connected together. R1 and R2 are meant for biasing the transistors. Collector
terminals of the two transistor are connected to the respective ends of the
primary of the output transformer T2. Power supply is connected between the
center tap of the T2 primary and the emitter junction of the Q1 and Q2. Base
terminal of each transistor is connected to the respective ends of the secondary of
the input coupling transformer T1. Input signal is applied to the primary of T1
and output load RL is connected across the secondary of T2.Quiescent current of
Q2 and Q1 flows in opposite directions through the corresponding halves of the
primary of T2 and as a result there will be no magnetic saturation. From the
figure you can see the phase splited signals being applied to the base of each
transistors. When Q1 is driven positive using the first half of its input signal, the
collector current of Q1 increases. At the same time Q2 is driven negative using the
first half of its input signal and so the collector current of Q2 decreases. From the
figure you can understand that the collector currents of Q1 and Q2 ie; I1 and I2
flows in the same direction trough the corresponding halves of the T2 primary.
As a result an amplified version of the original input signal is induced in the T2
secondary. It is clear that the current through the T2 secondary is the difference
between the two collector currents. Harmonics will be much less in the output
due to cancellation and this is results in low distortion.
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Class B push pull
The Class B push pull amplifier is almost similar to the Class A push pull
amplifier and the only difference is that there is no biasing resistors for a Class B
push pull amplifier. This means that the two transistors are biased at the cut off
point. The Class B configuration can provide better power output and has higher
efficiency (up to 78.5%). Since the transistors are biased at the cut-off point, they
consume no power during idle condition and this adds to the efficiency. The
advantages of Class B push pull amplifiers are, ability to work in limited power
supply conditions (due to the higher efficiency), absence of even harmonics in the
output, simple circuitry when compared to the Class A configuration etc. The
disadvantages are higher percentage of harmonic distortion when compared to
the Class A, cancellation of power supply ripples is not as efficient as in Class A
push pull amplifier and which results in the need of a well regulated power
supply.
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Q.9
What is complimentary symmetry amplifier?
Ans
Using complementary transistors (npn and pnp) it is possible to obtain a full cycle
output across a load using half-cycles of operation from each transistor.
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While a single input signal is applied to the base of both transistors, the
transistors, being of opposite 29type, will conduct on opposite half-cycles of the
input. The npn transistor will be biased into conduction by the positive half-cycle
of signal, with a resulting half-cycle of signal across the load . During the negative
half-cycle of signal, the pnp transistor is biased into conduction when the input
goes negative.
During a complete cycle of the input, a complete cycle of output signal is
developed across the load. One disadvantage of the circuit is the need for two
separate voltage supplies. Another, less obvious disadvantage with the
complementary circuit is shown in the resulting crossover distortion in the output
signal .Crossover distortion refers to the fact that during the signal crossover from
positive to negative (or vice versa) there is some nonlinearity in the output signal.
This results from the fact that the circuit does not provide exact switching of one
transistor off and the other on at the zero-voltage condition. Both transistors may
be partially off so that the output voltage does not follow the input around the
zero-voltage condition biasing the transistors in class AB improves this operation
by biasing both transistors to be on for more than half a cycle.
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Q.10 What is quasi complimentary symmetry amplifier?
Ans
A full and true "quasi complementary symmetry power amplifier" is an audio
amplifier whose power output section is typically comprised of 2 of the same
NPN output transistors, (generally matched) and 2 of the same PNP or NPN
driver transistors, (also generally matched). Driven by dual matching + & - DC
voltage power supply voltages. With the exception of the bias control circuit
components, each half of the output circuit are exact mirror images of one
another. Each half of the circuit amplifies the positive and negative half cycle of
the audio signal.
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Multiple Choice Questions
1.Flow of electrons is generally termed as _____________.
a) electric current
b) electric shock
c) semiconductor
d) none of the above
2.A _______________ is a material which offers very little resistance to theflow of current through
it.
a) good conductor
b) insulator
c) semiconductor
d) none of the above
3.The resistance offered by ______________ is extremely large for the flow of current through it.
a) good conductor
b) insulator
c) semiconductor
d) none of the above
4.The materials which behave like perfect insulators at low temperatures & athigher temperatures, they
behave like a good conductors are termed as ________.
a) good conductor
b) insulator
c) semiconductor
d) none of the above
5. The conductivity of a semiconductor _____________ with temperature.
a) increases
b) decreases
c) can’t say
d) none of the above
6. The conductivity of a good conductor _____________ with temperature.
a) increases
b) decreases
c) can’t say
d) none of the above
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7.The resistance of a semiconductor _____________ with temperature.
a) increases
b) decreases
c) can’t say
d) none of the above
8. The resistance of a good conductor _____________ with temperature.
a) increases
b) decreases
c) can’t say
d) none of the above
9. The charge of an electron is ___________________.
a) 1.602*10+27Coulomb
b) 1.602*10-27Coulomb
c) 1.602*10+19Coulomb
d) 1.602*10-19Coulomb
10.The total number of electrons in an atom depends upon ____________
a) the atomic mass
b) the atomic weight
c) the atomic number
d) the atomic size
11. The early effect in a bipolar junction transistor is caused by
(a) fast turn-on
(b)fast turn-off
(c) large collector-base reverse bias
(d) large emitter-base forward bias
12. MOSFET can be used as a
(a) current controlled capacitor
(b) voltage controlled capacitor
(c) current controlled inductor
(d) voltage controlled inductors
13. Thermal runaway is not possible in FET because as the temperature of FET increases
(a) the mobility decreases
(b) the transconductance increases
(c) the drain current increases
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(d) none of the above
14. A source follower using an FET usually has a voltage gain which is
(a) greater than +100
(b) slightly less than unity but positive
(c) exactly unity but negative
(d) about -10
15. A differential amplifier has a differential gain of 20,000 . CMRR=80 dB. The common
mode
gain is given by
(a) 2 (b) 1 (c) 1/2 (d) 0
16. The approximate input impedance of the OPAMP circuit which has
Ri=10k,Rf=100k,RL=10k
(a) ∞
(b)120k
(c)110k
(d)10k
17. An OPAMP has a slew rate of 5 V/μ S .The largest sine wave O/P voltage possible at
a
frequency of 1MHZ is
(a) 10 volts
(b) 5 volts
(c) vo5/lts
(d)5/2volts
18. A change in the value of the emitter resistance Re in a differential amplifier
(a) affects the difference mode gain Ad
(b) affects the common mode gain Ac
(c)affects both Ad and Ac
(d) does not effect either Ad and Ac
19. A differential amplifier is invariably used in the i/p stage of all OP-AMPs.This is
dome
basically to provide the OP-AMPs with a very high
(a)CMRR
(b)bandwidth
(c) slew rate
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(d)open-loop gain
20. The effective channel length of a MOSFET in a saturation decreases with increase in
(a) gate voltage
(b)drain voltage
(c)source voltage
(d)body voltage
21. Which of the following is not associated with a p-n junction
(a) junction capacitance
(b)charge storage capacitance
(c)depletion capacitance
(d)channel length modulation
22. In a p-n junction diode under reverse bias , the magnitude of electric field is
maximum at
(a) the edge of the depletion region on the p-side
(b) the edge of the depletion region on the n-side
(c) the p-n junction
(d) the center of the depletion region on the n-side
23. An n- channel JFET has IDSS=2mA,and Vp=-4v.Its transconductance gm=(in
mA/V)for an
applied gate to source voltage VGS=-2v is
(a)0.25
(b)0.5
(c)0.75
(d)1
24. In a common emitter, unbypassed resister provides
(a)voltage shunt feedback
(b)current series feedback
(c)negative voltage feedback
(d)positive current feedback
25. A constant current signal across a parallel RLC circuits gives an o/p of 1.4v at the
signal
frequency of 3.89KHZ and 4.1KHZ .At the frequency of 4KHZ,the o/p voltage will be
(a)1 v (b) 2v (c)1.4v (d)2.8v
26. Class AB operation is often used in power (large signal) amplifiers in order to
(a) get maximum efficiency
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(b)remove even harmonics
(c) overcome a crossover distortion
(d)reducing collector dissipation
27. The bandwidth of an RF tuned amplifier is dependent on
(a) Q –factor of the tuned o/p circuit
(b) Q –factor of the tuned i/p circuit
(c) Quiescent operating point
(d) Q-factor of the o/p and i/p circuits as well as quiescent operating point
28. If =0.98 ,Ico=6μA and Iβ=100μA for a transistor,then the value of Ic will be
(a)2.3mA
(b)3.2mA
(c)4.6 mA
(d)5.2mA
29.The MOSFET switch in its on-state may be considered equivalent to
(a)resistor
(b)inductor
(c)capacitor
(d)battery
30. Most of the linear ICs are based on the two-transistor differential amplifier because
of its
(a) input voltage dependent linear transfer characteristic
(b) high voltage gain
(c) high input resistance
(d) high CMRR
31. Negative feedback in an amplifier
a) Reduces gain
b) Increase frequency &phase distortion
c) Reduces bandwidth
d) Increases noise
32. A dc power supply has no-load voltage of 30v,and a full-load voltage of 25v at fullload
current of 1A.Its output resistance & load regulation ,respectively are
a) 5 Ω & 20 %
b) 25 Ω & 20 %
c) 5 Ω & 16.7 %
d) 25 Ω & 16.7 %
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33. The current gain of a bipolar transistor drops at high frequencies because of
a) Transistor capacitances
b) High current effects in the base
c) Parasitic inductive elements
d) The early effect
34. The ideal OP-AMP has the following characteristics.
a) Ri=∞,A=∞,R0=0
b) Ri=0 ,A=∞,R0=0
c) Ri=∞,A=∞,R0=∞
d) Ri=0 ,A=∞,R0=∞
35. A 741-type OP-AMP has a gain-bandwidth product of 1MHz.A non-inverting
amplifier
using this opamp & having a voltage gain of 20db will exhibit -3db bandwidth of
a) 50 KHz
b) 100KHz
c) 1000/17 KHz
d) 1000/7.07 KHz
36.An amplifier using an opamp with slew rate SR=1v/μsec has a gain of 40db.if this
amplifier
has to faithfully amplify sinusoidal signals from dc to 20 KHz without introducing any
slew-rate
induced distortion, then the input signal level must not exceed
a) 795mV
b) 395mV
c) 795 mV
d) 39.5mV
37. In the differential voltage gain & the common mode voltage gain of a differential
amplifier
are 48db &2db respectively, then its common mode rejection ratio is
a)23dB
b)25dB
c) 46dB
d) 50dB
38. Generally, the gain of a transistor amplifier falls at high frequencies due to the
a) Internal Capacitance of the device
b) Coupling capacitor at the input
c) Skin effect
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d) Coupling capacitor at the output
39. An amplifier without feedback has a voltage gain of 50,input resistance os 1 KΩ &
Output
resistance of 2.5KΩ.The input resistance of the current-shunt negative feedback amplifier
using
the above amplifier with a feedbacik factor of 0.2 is
a) 1/11KΩ
b) 1/5KΩ
c) 5KΩ
d) 11KΩ
40. The action of JFET in its equivalent circuit can best be represented as a
a) Current controlled Current source
b) Current controlled voltage source
c) Voltage controlled voltage source
d) voltage controlled current source
41. Three identical amplifiers with each one having a voltage gain of 50,input resistance
of 1KΩ
& output resistance of 250,are cascaded.The open circuit voltage gain of combined
amplifier is
a) 49dB
b) 51dB
c) 98dB
d) 102dB
42. An ideal OP-AMP is an ideal
a) Current controlled Current source
b) Current controlled voltage source
c) Voltage controlled voltage source
d) voltage controlled current source
43. In a full-wave rectifier using two ideal diodes,Vdc& Vm are the dc & peak values of
the
voltage respectively across a resistive load. If PIV is the peak inverse voltage of the
diode, then
the appropriate relationships for this rectifier is.
a) Vdc = Vm/π, PIV=2Vm
b) Vdc = 2Vm/π, PIV=2vm
c) Vdc = 2Vm/π, PIV=Vm
d) Vdc = Vm/π, PIV=Vm
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44. The cascade amplifier is a multistage configuration of
a) CC-CB .
b) CE-CB
c) CB-CC
d) CE-CC
45. The most commonly used amplifier in sample & hold circuits is
a) A unity gain non-inverting amplifier
b) A unity gain inverting amplifier
c) An inverting amplifier with a gain of 10
d) An inverting amplifiers with a gain of 100
46. Assume that the op-amp of the fig. is ideal. If Vi is a triangular wave ,then V0will be
C RV-V0
a) square wave
b) Triangular Wave
C) Parabolic Wave
d) Sine Wave
47. Introducing a resistor in the emitter of a common amplifier stabilizes the dc
operating point
against variations in
a) Only the temperature
b) only the β of the transistor
c) Both Temperature & β
d) None of the above
48. Voltage Series feedback (also called series-shunt feedback) results in
a) Increase in both input & output impedances
b) Decreases in both input & output impedances
c) Increase in input impedance & decreases in output impedance
d) Decrease in input impedance & increase in output impedance
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Ans:- 1.(a), 2(a), 3(b) ,4(c) ,5(a), 6(b) ,7(b) ,8(a) ,9(d), 10(c),11. (c),1 2. (b), 13. (a), 14. (a), 15.
(a), 16. (d), 17. (d), 18. (b), 19. (a), 20. (b), 21. (d), 22. (c), 23. (b), 24. (c), 25. (b), 26. (c), 27.
(a), 28. (d), 29. (c), 30. (d), 31. (a),32. (b), 33. (a), 34. (a), 35. (b), 36. (c), 37. (c), 38. (a), 39.
(a), 40. (d), 41. (c), 42. (b), 43. (b), 44. (b), 45. (b), 46. (c), 47. (c), 48. (c)
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Bibliography
1. M. H. Rashid, Microelectronic Circuits Analysis and Design, Cengage Learning.
2. Millman, Integrated Electronics, TMH.
3. A. S. Sedra, Kenneth C. Smith, Microelectronic Circuits, Oxford University Press.
4. Fundamentals of Analog Circuits 2e, Floyd, Pearson
5. David A. BELL, Electronic Devices and Circuits, Oxford University Press.
6. Electronic Devices and Circuits–II, R.Tiwari, Genius publications
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